Sequential high-rate charging of battery cells

ABSTRACT

In a preferred embodiment, a sequential high-rate battery charger for charging a battery having a plurality of cell units, the battery charger including a current source; an apparatus to selectively sequentially connect each of the plurality of cell units to the high current source; and an apparatus to sequentially provide high current from the high current source to each of the plurality of cell units for a first selected period of time.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority of International PatentApplication No. PCT/US97/12527, filed Jul. 17, 1997, and claims benefitof the filing date of U.S. Provisional Application Ser. No. 60/021,984,filed Jul. 18, 1996.

TECHNICAL FIELD

The present invention relates to battery charging generally and, moreparticularly, but not by way of limitation, to novel means and methodfor sequentially charging battery cells or cell banks using a high rateof current.

BACKGROUND ART

The rapid charging of batteries, and in particular lead-acid batteries,has been pursued for decades. Procedures for rapid charging of batterieshave been reported over the past 20 years or so, mainly for Ni—Cdbatteries and, to a lesser extent, for lead-acid batteries. Interest inthe latter has increased lately because of intensified interest inelectric street vehicles.

All properly designed batteries contain more active material in theirplates than their ratings would indicate. In spite of this, mostsecondary (rechargeable) batteries, especially lead-acid batteries, areroutinely used to only about 80 percent of their ratings. Even thoughonly about 80 percent of rated capacity is extracted, the cycle life andlifetime energy throughput is significantly reduced from that ofshallower discharges. The cycle life and lifetime energy throughput at100 percent depth-of-discharge is typically very low.

Conventional charging techniques coupled with rigorous standarddischarge conditions often yield a significant amount of cycle-to-cyclecapacity variation. Furthermore, grain structure of the active platematerial becomes worse and worse with each charging cycle.

A significant amount of research has recently focused on high-ratecharging, primarily for rapid recharge (usually partial recharge) forextended range or emergency conditions in street electric vehicles.Until recently, battery charging designers followed the “ampere-hourrule” which holds that the rate of recharge current at any point in thecharging cycle should equal the number of ampere hours to be recharged.In spite of this “rule,” remarkable side benefits have emerged fromstudies of high-rate charging—that is, charging rates greatly in excessof that prescribed by the “ampere-hour rule.” It appears from theresults of these studies that high-rate charging permits greaterutilization of active plate material which allows greaterdepths-of-discharge without detrimental effects and, in fact, is oftenaccompanied by significantly greater cycle life and lifetime energythroughput.

Preliminary physical analyses of high-rate charging effects show: (1)improved maintenance of optimum crystal size within the plate structure,(2) better penetration or use of depth into the third dimension ofactive plate material, (3) increased electrolyte stirring and convectionwithin local regions and throughout the plate and electrolyte reservoirchannels, and (4) enhanced nucleation for crystal formation in deficientplate regions.

Conventional high-rate charging has an objective charging a battery in asmall fraction of the time required for conventional charging, throughthe application of a high rate of current in parallel to all batterycells or cell blocks. Accordingly, a primary drawback of conventionalhigh-rate charging is the extremely high power inputs required. Forexample, a charger might require 5 kilowatts (220 VAC@23 amperes) duringthe early part of the charging cycle for conventional charging, butcould easily require 50 to 100 kilowatts (440 VAC@113 to 228 amperes)for high-rate charging. Another problem with conventional high-ratecharging techniques, which affects battery life, is the inevitablepolarization and its concomitant voltage gradients and overvoltages,which usually requires periodic equalization between cells or cellbanks. Ideally, some form of equalization should occur during eachrecharge, but this is usually impractical. Further problems withconventional high-rate charging are a high rate of temperature increaseand the possibility of dangerous pressure increase.

Accordingly, it is a principal object of the present invention toprovide means and method for high-rate charging that do not necessarilyrequire exceeding the maximum power requirements of conventional (i.e.,not high-rate) charging methods.

It is a further object of the invention to provide such means and methodthat can permit complete charging in less time than conventionalcharging techniques.

It is an additional object of the invention to provide such means andmethod that permit utilization of a greater percentage of rated batterycapacity, ideally 100 percent of a well designed battery.

It is another object of the invention to provide such means and methodthat increase cycle life and lifetime energy throughput, to perhaps adoubling or tripling of cycle life and lifetime energy throughput.

A further object of the invention is to provide such means and methodwhich performs equalization (cell-by-cell or block-by-block) during eachrecharge.

An additional object of the invention is to provide such means andmethod that improve overall charge coulombic efficiency from, say, 90percent for conventional charging to greater than 95 percent.

Another object of the invention is to provide such means and method thatoptimize and synchronize the battery state-of-charge computer employed.

Yet a further object of the invention is to provide such means andmethod that provide alerts concerning abnormalities, especiallyequalization imbalance.

Other objects of the present invention, as well as particular features,elements, and advantages thereof, will be elucidated in, or be apparentfrom, the following description and the accompanying drawing figures.

DISCLOSURE OF INVENTION

The present invention achieves the above objects, among others, byproviding, in a preferred embodiment, a sequential high-rate batterycharger for charging a battery having a plurality of cell units, saidbattery charger comprising: a high current source; means to selectivelysequentially connect each of said plurality of cell units to said highcurrent source; and means to sequentially provide high current from saidhigh current source to said each of said plurality of cell units for afirst selected period of time.

BRIEF DESCRIPTION OF DRAWINGS

Understanding of the present invention and the various aspects thereofwill be facilitated by reference to the accompanying drawing figures,submitted for purposes of illustration only and not intended to definethe scope of the invention, on which:

FIG. 1 is a block/schematic diagram showing a battery and batterycharging system, the battery being connected to a system load.

FIG. 2 is a timing diagram for the battery charging system of FIG. 1.

FIG. 3 is a plot of current vs. time for a conventional charging cycleand a charging cycle according to the present invention.

FIG. 4 is a plot of coulombic efficiency vs. percent state-of-charge fora conventional charging cycle and a charging cycle according to thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Reference should now be made to the drawing figures, on which similar oridentical elements are given consistent identifying numerals throughoutthe various figures, and on which parenthetical references to figurenumbers direct the reader to the view(s) on which the element(s) beingdescribed is (are) best seen, although the element(s) may be seen alsoon other views.

FIG. 1 illustrates a battery charging system, constructed according tothe present invention, and generally indicated by the reference numeral20, connected to charge a battery generally indicated by the referencenumeral 22, the battery being connected to a system load 24.

Battery 22 includes twelve cell units, CB1 through CB12, which may beindividual cells or cell blocks. In the present case, it will be assumedthat battery 22 is a 144-volt, 125-amperehour, twelve-cell-block(12×6×2v/cell), system. Charging system 20 includes twelve switch pairs,S1 through S12, each connected to provide charging current to one ofcell units CB1 through CB12 from a high current source 30. A sequencer32 is connected to operate switches S1 through S12 and the sequencer andan analog-to-digital converter and “topping up algorithm” block 34 areconnected to high current source 30 and to a battery state-of-chargecomputer and diagnostician 36. Battery state-of-charge computer anddiagnostician 36 is connected to receive a system battery currentindication input from across a system shunt 38 connected in series withbattery 22 and is also connected via a cable 40 to receive internaltemperatures of cell units CB1 through CB12. Cable 40 is also connectedto provide temperature inputs to digital-to-analog converter and“topping up” algorithm block 34.

In operation, the general scheme of the present invention is tosequentially charge cell units CB1 through CB12, one at a time, with acharging current that is high compared with that which would be appliedto an individual cell during conventional charging of battery 22. Thesequence cycle is repeated, as necessary, until battery 22 is fullycharged. As is indicated below, the total charging time can be less thanthe charging time for conventional techniques, while reaping thebenefits derived from high-rate charging, although the charging currentat any point in the cycle is less than the peak current required forconventional charging. Further benefits will also be described.

Reference now also to FIG. 2 will aid in understanding the operation ofthe present invention. FIG. 2 shows an example of a timing cycle for thefirst three cell units CB1 through CB3 of battery 22 and it will beunderstood that the timing cycle is identical for the remainder of thecell units of the battery.

In the first 1 millisecond interval, a closing signal is supplied to S1(FIG. 1) from sequencer 32 to close S1 to connect high current source 30to cell unit CB1. Four milliseconds later, high current source 30applies charging current to cell unit CB1.

In the 4 millisecond interval between application of the closing signalto S1 and current flow from high current source 30, analog-to-digitalconverter 34 measures the condition (open circuit voltage) of cell unitCB1. Charging current is then applied to cell unit CB1 for 59.94seconds, during which charging interval analog-to-digital converter 34measures the voltage response of cell unit CB1. After the charginginterval, there is a 55 millisecond interval for recovery delay andsafeguard delay and for analog-to-digital converter 34 to measure thecondition of cell unit CB1. The latter is the first conditionmeasurement of CB1 during the relaxation portion of the cycle for cellunit CB1, the second condition measurement being made just before thenext charging interval for that cell unit. The condition of cell unitCB1 could also be measured continuously during the relaxation portion byadding connections between cell units CB1 through CB12 andanalog-to-digital converter 34, such as connection 42 between cell unitCB1 and the analog-to-digital converter.

Following the above 55 millisecond interval, a closing signal issupplied to S2 (FIG. 1) from sequencer 32 and the charging of cell unitCB2 is identical to that described above for cell unit CB1. Likewise,cell unit CB3 and the other cell units CB4 through CB 12 are similarlycharged sequentially.

Summarizing the timing cycle for a cell unit:

Switching  1 ms Stabilize  1 ms OCV A-D convert  2 ms Safeguard delay  1ms  5 ms Current on 59.94 s* Recovery delay 52 ms OCV A-D convert  2 msSafeguard delay  1 ms 55 ms *Closed circuit voltage measured during thisinterval.

Thus, it is seen that a full round trip charging cycle for the twelvecell units CB1 through CB12 is 12 minutes and that switching andmeasurement occur in 0.1 percent (60 ms) of a total 60 second chargingcycle for a cell unit. Thirty-two such charging cycles are required fora 6.4-hour recharge. Initially, current is flowing for 99.9 percent ofthe time.

FIG. 3 compares the average current versus time over 6.4- and 8-hourcharging intervals for, respectively, the above charging system and aconventional charging system, with the same peak line powerrequirements.

Total ampere-hours (AH) during charge may be determined as follows:

AH=C×U/E,

where,

C=nominal battery capacity,

U=% capacity utilization, and

E=net charge coulombic efficiency.

Thus, for conventional charging:

AH=125×0.80/0.90=111.1,

and for the above sequential charging:

AH=100×1.00/0.97=103.1,

Current “on” intervals are expected to be practical over the range ofseveral seconds to several minutes, but the optimum is expected to be onthe order of one-half to three minutes, thus leading to the one minuteexample of FIG. 2.

It is assumed that switches S1 through S12 are inexpensive, slow solidstate switches, although fast switches may be employed instead.Electromechanical switches may be substituted, with the allowance oflonger switching times. It can be seen that all high power switching isdone under zero current conditions. With only one switch pair active atany one time, the switching array can share drive sources and heat sinkswith little or no capacity beyond that required for a single pair.

The technique of the present invention provides extremely high coulombicefficiency by minimizing polarization effects because, using the aboveexample, of the short “on” time (59.94 seconds) and the long “off” time(660.060 seconds). The long relaxation period guarantees that cell unitsCB1 through CB12 have a long time to chemically and thermallyequilibrate and, thus, equalization inherently takes place withoutfurther action. The mitigation of polarization provided by the longrecovery time as the battery approaches full charge delays the finalthrottling of current which is inevitably required during “topping up.”Such throttling may be in terms of time or current. In the aboveexample, the 59.94 seconds of charging can be considered a maximum, witha shorter period of charging provided during “topping up.” The longrelaxation period is also long enough that some rather precisecalculations can be made to determine the state-of-charge of battery 22and its health.

It is well known that the coulombic efficiency is inherently high duringcharge at low states-of-charge, even with conventional low-rate chargingtechniques. Using the technique of the present invention, the very highefficiency region is: (1) driven lower, (2) is at least as high atnormal low states-of-charge, and (3) extends to higher than normalstates-of-charge. The recharge coulombic efficiency profiles over, say,an eight-hour charge span could approximate the contours shown on FIG.4. In summary: the conventional system (assuming 80 percent capacityutilization) provides an overall charge coulombic efficiency of 90.3percent, while the technique of the present invention (assuming 100percent capacity utilization) provides an overall charge coulombicefficiency of 97.3%.

The peak sequential charge current is approximately 12 times the averagesequential charge current, but the peak sequential charge power is thesame as the “peak” steady state conventional charge power.

Details of power equivalencies for the above comparison are as follows:

Conventional starting current=26 A.

Battery voltage at start of charge=150 V.

Power delivered to battery=150×26=3900 W.

Power conversion efficiency=92% for sequential.

Line power=3900/0.92=4239.1 W input power.

Power conversion efficiency of sequential=88%, because of low voltage,high current.

Power delivered to cell block=0.88 (4239.1)=3730.4 W, because of highcurrent.

Cell block voltage later in charging process=2.7(6)=16.2V.

Peak current allowed=3730.4W/16.2V=230.3A.

Average current=0.999(1/12)(230.3)=19.17 A.

While a feature of the present invention is that charging power can belimited to line power inputs equivalent to conventional chargers, suchis not a required limitation. For example, the charging time of theabove example could be nearly halved to, say, 3½ to 3¾ hours by doublingthe peak and average currents. Working the problem in reverse andassuming somewhat lower efficiency and higher cell block voltages yieldsthe following input power (line power):

Average current=19.17(2)=38.34 A.

Peak current=1/0.999(12)(38.34)=460.55 A.

Cell block voltage later in charging process=2.85(6)=17.1 V.

Power delivered to cell block=(460.55)(17.1)=7875.4 W.

Line power=7875.4/0.86=9157.4 W.

Temperature measurement is desirable, both from the aspect of ambientbattery temperature and internal temperature rise during the chargingprocess. Electrochemically, the sequential charging technique is veryefficient, but I²R losses will be greater than steady lower currentcharging. As indicated above, temperature is measured in each cell unitCB1 through CB12. This information is used in a number of ways, asfollows:

(1) As an input to the “topping up” algorithm 34 which is refined toexamine “on” voltage, “on” dv/dt, “off” voltage, “off” dv/dt, andtemperature. This algorithm instructs the high current source to reduce“on” time and/or reduce current amplitude as full charge is approached.The purpose of a minimum of two measurements in both the current “on”and relaxation portions of a cycle is to obtain both absolute value andrate of change information, in order to enhance the precision ofstate-of-charge estimation and subsequently to provide throttlinginstructions during “topping up.” A particular battery will usually bemodeled and tested to provide the basis for current, voltage, andtemperature correlation.

(2) As inputs to the battery state-of-charge computer 36 to improve theaccuracy of the computation. During charging, battery state-of-chargecomputer 36 will recharge pessimistically, showing the net batterystatus as a function of the lagging cell unit. During discharging, asingle representative cell unit, or several cell units, or an average ofall cell units, can serve as input(s) to the discharge side of thebattery state-of-charge computation.

(3) As inputs to the diagnosis portion of state-of-charge computer 36relative to abnormal temperature and abnormal cell unit-to-cell unitcomparisons. Such comparisons are used, for example, to provide alertsconcerning abnormalities, especially equalization imbalances. Anotherabnormality is that one or more of cell units CB1 through CB12 is (are)not throttling back which indicates problems with the cell unit(s).

“Topping up” during throttling may result in a particular cell unit CB1through CB12 receiving more or less charging current (or time) thanothers of the cell units. In some case, one or more cell units CB1through CB12 may receive no additional charging during a charging cycle,depending on voltage information taken immediately before what wouldotherwise be a period of charging.

Weight savings may also be realized through the use of the sequentialcharging technique. For example, assuming: (1) 80 percentdepth-of-discharge for a conventional system, (2) 100 percentdepth-of-discharge for a sequential system, (3) 18 watt-hours per poundfor a state-of-the-art lead-acid battery energy density, and (4) a12-step switching matrix, the weight savings are calculated as follows:

Conventional = 125 AH(144 V) = 1000 pounds 18 WH/Pound Sequential =100 AH(144 V) =  800 pounds 18 WH/Pound Additional Weight of Matrix = 40 pounds Additional Weight of Circuitry =  10 pounds Net Savings = 150 pounds

Similar savings in volume can be expected as well.

It will thus be seen that the objects set forth above, among thoseelucidated in, or made apparent from, the preceding description, areefficiently attained and, since certain changes may be made in the aboveconstruction without departing from the scope of the invention, it isintended that all matter contained in the above description or shown onthe accompanying drawing figures shall be interpreted as illustrativeonly and not in a limiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

What is claimed is:
 1. A sequential battery charging apparatus for abattery having a plurality of cell units, said apparatus comprising: (a)a current source; (b) a controller connected to said current source andto each of said plurality of cell units; (b) said controller including asequencer to sequentially remove each of said plurality of cell unitsfrom service and to provide current from said current source to saideach of said plurality of cell units, while removed from service, for afirst selected period of time; and (c) level of charging current duringa recharging cycle of said plurality of cell units remaining,substantially throughout said recharging cycle, near peak chargingcurrent required for a steady-state charging cycle of said plurality ofcell units, for charging cycles of substantially equal lengths.
 2. Asequential battery charging apparatus, as defined in claim 1, furthercomprising: means to sequentially remove said current source from saidplurality of cell units for a second period of time following said firstperiod of time, said second period of time being substantially greaterin length than said first period of time.
 3. A sequential batterycharging apparatus, as defined in claim 1, further comprising: means tosequentially determine voltage condition of said plurality of cell unitsduring said first period of time.
 4. A sequential battery chargingapparatus, as defined in claim 1, further comprising: means tosequentially determine voltage condition of said plurality of cell unitsat least once during said second period of time.
 5. A sequential batterycharging apparatus, as defined in claim 4, further comprising: means tosequentially determine voltage condition of said plurality of cell unitstwice during said second period of time to determine rate of change ofsaid voltage condition during said second period of time.
 6. Asequential battery charging apparatus, as defined in claim 5, wherein:said means to sequentially determine voltage condition during saidsecond period of time determines said voltage condition near beginningand end of said second period of time.
 7. A sequential battery chargingapparatus, as defined in claim 1, wherein: total time to charge saidbattery with said sequential battery charging apparatus is no greaterthan total time to charge said battery using said steady-state chargingcycle for the same degree of charging.
 8. A sequential battery chargingapparatus, as defined in claim 1, wherein: peak current to charge saidbattery with said sequential battery charging apparatus in a givenperiod of time is less than peak current to charge said battery usingsaid steady-state charging cycle in said given period of time.
 9. Asequential battery charging apparatus, as defined in claim 1, furthercomprising: abnormality alerting means connected to said plurality ofcell units to signal an abnormality in at least one of said plurality ofcell units based on voltage measurements of said at least one of saidplurality of cell units.
 10. A sequential battery charging apparatus, asdefined in claim 9, further comprising: means connected to saidplurality of cell units to provide temperature inputs to saidabnormality alerting means.
 11. A sequential battery charging apparatus,as defined in claim 1, wherein: said battery charging apparatus providesgreater depth-of-discharge for said battery compared with saidsteady-state charging cycle for the same degree of charging.
 12. Asequential battery charging apparatus, as defined in claim 1, wherein:said battery charging apparatus provides a greater number of dischargecycles for said battery compared with said steady-state battery chargingcycle for the same degree of charging.
 13. A sequential battery chargingapparatus, as defined in claim 1, wherein: said battery chargingapparatus provides inherent equalization of said cell units.
 14. Asequential battery charging apparatus, as defined in claim 1, wherein:said battery charging apparatus provides greater coulombic efficiencycompared with said steady-state charging cycle for the same degree ofcharging.
 15. A method of sequentially charging, with a current source,a battery having a plurality of cell units, said method comprising: (a)selectively sequentially connecting each of said plurality of cell unitsto said current source; (b) sequentially providing current from saidcurrent source to said each of said plurality of cell units for a firstselected period of time; and (c) level of current during a rechargingcycle of said plurality of cell units remaining, substantiallythroughout said recharging cycle, near peak charging current requiredfor a steady-state charging cycle of said plurality of cell units forcharging cycles of substantially equal lengths.
 16. A method, as definedin claim 15, further comprising: sequentially removing said currentsource from said plurality of cell units for a second period of timefollowing said first period of time, said second period of time beingsubstantially greater in length than said first period of time.
 17. Amethod, as defined in claim 15, further comprising: sequentiallydetermining voltage condition of said plurality of cell units duringsaid first period of time.
 18. A method, as defined in claim 16, furthercomprising: sequentially determining voltage condition of said pluralityof cell units at least once during said second period of time.
 19. Amethod, as defined in claim 18, further comprising: sequentiallydetermining voltage condition of said plurality of cell units twiceduring said second period of time to determine rate of change of saidvoltage condition during said second period of time.
 20. A method, asdefined in claim 19, wherein: sequentially determining voltage conditionduring said second period of time determines said voltage condition nearbeginning and end of said second period of time.
 21. A method, asdefined in claim 15, further comprising: charging said battery in atotal time no greater than total time to charge said battery using saidsteady-state charging cycle.
 22. A method, as defined in claim 15,further comprising: charging said battery in a given period of time witha peak current less than peak current to charge said battery using saidsteady-state charging cycle in said given period of time.
 23. A method,as defined in claim 15, further comprising: signaling an abnormality inat least one of said plurality of cell units based on voltagemeasurements of said at least one of said plurality of cell units.
 24. Amethod, as defined in claim 23, further comprising: providingtemperature inputs of said cell units to affect signaling saidabnormality.
 25. A method, as defined in claim 15, wherein: said methodprovides greater depth-of-discharge for said battery compared with saidsteady-state charging cycle for the same degree of charging.
 26. Amethod, as defined in claim 1, wherein: said method provides a greaternumber of discharge cycles for said battery compared with saidsteady-state charging cycle for the same degree of charging.
 27. Amethod, as defined in claim 1, wherein: said method provides inherentequalization of said cell units.
 28. A method, as defined in claim 1,wherein: said method provides greater coulombic efficiency compared withsaid steady-state charging cycle for the same degree of recharging.